This invention relates to cardiac stimulating devices and particularly to implantable cardiac stimulating devices capable of providing rate-responsive pacing therapy. More particularly, this invention is directed toward a miniature, accelerometer-based, physical activity sensor particularly adapted to be mounted within such devices, for measuring levels to which a patient is engaged in physical activity, so that rate-responsive pacing therapy may be administered accordingly.
A pacemaker is a type of implantable medical device which delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia--a condition in which a patient cannot normally maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, which was typically set at a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, and in more advanced pacemakers, could be set remotely after implantation.
Early advances in pacemaker technology included the ability to sense intrinsic cardiac activity of a patient (i.e., the intercardiac electrogram, or "IEGM"), which led to the development of "demand pacemakers," so named because stimulation pulses were provided only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous, hemodynamically effective, cardiac contraction which occurs within a predetermined time period (commonly referred to as the "escape interval") following a preceding contraction. When a naturally occurring contraction is detected within the escape interval, a demand pacemaker does not deliver a pacing pulse. The ability of demand pacemakers to avoid delivery of unnecessary stimulation pulses is desirable, because it extends battery life.
Early demand pacemakers enabled a physician to adjust the heart rate to be maintained by telemetrically adjusting the length of the escape interval. However, this flexibility only allowed for adjustments to a fixed "programmed rate," and did not accommodate patients who required increased or decreased heart rates to meet changing physiological requirements during periods of elevated or reduced physical activity. Therefore, unlike a person with a properly functioning heart, these patients were paced so that a constant heart rate was maintained regardless of the level to which the patient was engaged in physical activity. Thus, during periods of elevated physical activity, these patients were subject to adverse physiological consequences, including lightheadedness and episodes of fainting, because their heart rates were forced by the pacemaker to remain constant.
Later pacemakers were capable of adjusting the rate at which pacing pulses are delivered in accordance with metabolic needs of the patient. These devices, known as "rate-responsive pacemakers," typically maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increase the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Rate-responsive pacemakers typically include circuitry that correlates measured physical activity to a desirable heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and the slope or curve between the minimum heart rate and the maximum heart rate are telemetrically programmable to meet the needs of a particular patient.
One approach that has been considered for correlating physical activity to an appropriate heart rate involves measuring a physiological parameter that reflects the level to which the patient is engaged in physical activity. Physiological parameters that have been considered include central venous blood temperature, blood pH level, QT time interval and respiration rate. However, certain drawbacks such as slow response time, excessive emotionally induced variations, and wide variability across individuals, render the use of certain physiological parameters difficult, and accordingly, they have not been widely applied in practice.
More generally accepted have been rate-responsive pacemakers which employ sensors that transduce mechanical forces associated with physical activity. A widely used type of sensor incorporates a piezoelectric crystal which generates a measurable electrical potential when a mechanical stress resulting from physical activity is applied to the sensor. U.S. Pat. No. 4,140,132 (to Dahl) and U.S. Pat. No. 4,428,378 (to Anderson et al.) describe examples of rate-responsive pacemakers that maintain a paced heart rate in accordance with physical activity as measured by a piezoelectric sensor.
Despite the widespread use of piezoelectric sensors in rate-responsive pacemakers for measuring physical activity, certain difficulties remain which have yet to be overcome. For example, sensors that employ piezoelectric crystals typically provide extremely small output signals, and subsequent signal processing is often difficult. The small output signals provided by these sensors are usually the result of design choices which are made to compensate for the fragility of the crystals. More precisely, piezoelectric crystals are known to be extremely brittle and subject to fracturing if excessively stressed. To prevent fracturing while in use, sensors must be designed so that relatively high levels of physical exertion by patients do not cause stresses that are beyond the tolerance limits of the crystals. However, the output signals provided by piezoelectric sensors are directly proportional to the magnitude of the mechanical stresses experienced by the piezoelectric material. Thus, to ensure that these known sensors function properly over the lifetime of the pacemaker, the strength of the output signals provided by many of these devices is sacrificed to some extent.
The fragility of piezoelectric crystals also presents certain difficulties during the fabrication process. First, the process of assembling a sensor incorporating a piezoelectric crystal is difficult because handling of the piezoelectric crystal during sensor assembly can cause stresses which exceed the tolerance limits of the crystal. Also, the process of securing the sensor to a suitable supporting structure in the pacemaker can cause unacceptably high stresses. Thus, the fabrication process for pacemakers which incorporate physical activity sensors that rely on piezoelectric crystals may require more expensive equipment and time-consuming procedures than would otherwise be desirable.
Piezoelectric sensors that are constructed in the form of a weighted cantilever beam, such as the sensor described in U.S. Pat. No. 4,140,132 (to Dahl), present further difficulties during the fabrication process. Typically, these sensors are not free-standing; therefore, unless they are secured to a suitable supporting structure, they may tip over. The impact of a sensor on a rigid material can cause the sensor to experience a stress of significant magnitude and as described above, a fracture may result. During the fabrication of a pacemaker, the sensor is usually adhered to a supporting structure with a suitable epoxy. However, the epoxy usually cures very slowly, so the possibility of tipping and subsequent breakage is great, unless additional precautions, such as the insertion of a shim, are used to restrain the cantilever beam from tipping until the epoxy has cured. Of course, the use of additional precautions is undesirable, since they add complexity to the fabrication process.
Another concern regarding piezoelectric physical activity sensors relates to the size and number of components required to construct the sensors. There is tremendous demand for implantable cardiac stimulating devices of reduced size but increased functionality. Many piezoelectric physical activity sensors, especially those of a cantilever beam design, require supporting members for anchoring the sensor to a suitable substrate, as well as a pair of electrical contacts for conducting an output signal provided by the sensor to circuitry within the pacemaker. It is often difficult to accommodate these components within the confines of a pacemaker of acceptable size, and accordingly, it would be desirable to reduce to the greatest extent possible the size and number of components necessary to implement a physical activity sensor.
What is needed therefore is an improved physical activity sensor suitable for use with a rate-responsive pacemaker that overcomes the deficiencies associated with the prior art sensors described above. The improved sensor should provide a relatively strong output signal and should be manufacturable in an efficient and cost-effective manner. Ideally, the 10 sensor should be easy to secure to a substrate in the pacemaker. More particularly, the sensor should be mountable to the pacemaker hybrid, so that the assembly and installation of the sensor can be conveniently integrated to the hybrid manufacturing process. The sensor should also be resistant to breakage, both during fabrication and in use. Finally, the sensor should be compact, both in terms of its overall dimensions and the size and number of individual components.